- A
The private key must be kept secret by the user
The private key should never be shared or stored on the server.
- B
The public key is used to decrypt the session
Why wrong: The public key is used to encrypt a challenge that only the private key can decrypt.
- C
Public-key authentication does not require a passphrase
Why wrong: Private keys can be protected with a passphrase, though not required.
- D
The private key is stored on the server
Why wrong: The private key must remain on the client; the public key is stored on the server.
Question 788 of 1,000
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company uses SSH for remote administration. To enhance security, they want to implement public-key authentication. Which statement about SSH public-key authentication is true?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The private key must be kept secret by the user
In SSH public-key authentication, the private key is the secret half of the asymmetric key pair and must be kept confidential by the user. The server stores only the public key, and authentication is proven by the client signing a challenge with the private key, which the server verifies using the stored public key. This ensures that even if the server is compromised, the private key remains safe on the client side.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The private key must be kept secret by the user
Why this is correct
The private key should never be shared or stored on the server.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The public key is used to decrypt the session
Why it's wrong here
The public key is used to encrypt a challenge that only the private key can decrypt.
- ✗
Public-key authentication does not require a passphrase
Why it's wrong here
Private keys can be protected with a passphrase, though not required.
- ✗
The private key is stored on the server
Why it's wrong here
The private key must remain on the client; the public key is stored on the server.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the roles of public and private keys, mistakenly thinking the public key is used for decryption or that the private key is stored on the server, when in fact the private key is kept secret by the user and used only for signing.
Trap categories for this question
Keyword trap
Private keys can be protected with a passphrase, though not required.
Detailed technical explanation
How to think about this question
Under the hood, SSH public-key authentication uses the private key to sign a session-specific challenge (e.g., the server's host key and session ID) using algorithms like RSA or Ed25519. The server verifies this signature against the public key in ~/.ssh/authorized_keys, proving possession of the private key without ever transmitting it. A subtle behavior is that SSH can also use the private key for host-based authentication, but for user authentication, the private key remains client-side only.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this CISSP question test?
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The private key must be kept secret by the user — In SSH public-key authentication, the private key is the secret half of the asymmetric key pair and must be kept confidential by the user. The server stores only the public key, and authentication is proven by the client signing a challenge with the private key, which the server verifies using the stored public key. This ensures that even if the server is compromised, the private key remains safe on the client side.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
This CISSP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISSP exam.
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